The idea of producing solar energy in the desert appears, at first glance, quite appealing. There is a lot of sunshine available, and annual fluctuations in productivity are negligible compared to locations further up north. The main problem is just to transport the electricity generated in the desert to the consumers who, generally, are not residing at the place of production.
Desertec is an ambitious project aiming at producing large amounts of (mainly solar) energy in the desert and transmitting it across the Mediterranean to the consumers in Europe. The anticipated transfer volume is expected to rise from 60 TWh per year in 2020 to about 700 TWh in 2050. The latter figure corresponds roughly to 20% of all electricity generated in the 27 EU Member States in 2007. So there is indeed a substantial amount of energy available in the desert sun which was clear from the outset.
The crucial problem we are facing here is the following: transporting energy requires energy, and the longer the transport route the more energy you need for transporting, i.e. the higher the losses. Even with the best available technology we would expect to get less electricity out of the socket than what has been put in at the beginning of the transmission line.
High-voltage direct current (HVDC) is currently the best available technology for transmitting electricity over very long distances. The losses amount to a minimum of 3% per 1000 km. Thus, for a transfer volume of 60 TWh we would expect to lose some 1.8 TWh over 1000 km. Desertec estimates the length of its transmission lines to be more than 3000 km in 2020. In case that we would encounter transmission losses of at least 5.4 TWh per year. Taking into account the anticipated transfer volume and the expected length of the transmission network for 2050 we can calculate the power losses to be at least of the order of 75 TWh, i.e. more than the entire transfer forecast for 2020.
To put things into perspective, the anticipated transmission losses for the Desertec project correspond to the annual output of 7 nuclear power stations (taking the Isar 2 power plant as a reference which, as the top performing German nuclear plant, produces an average of 11 TWh per year).
Thus, apart from worries about the policitcal stability of the producing region and the related issue of security of supply which are clearly outside the scope of this posting, transmitting energy across the Mediterranean sea is facing technical limitations which, in their entirety, may add up to considerable factors. From a technical point of view, it is far from evident, that producing solar energy in the desert and pushing it over the sea to Europe is a smart way of doing things. It is certainly more promising to increase the number of solar power facilities in Europe and connect them to local grids rather than looking for a far-fetched solution.